The present invention relates to lighting devices and, more particularly, to lighting devices that include a semiconductor light emitting device and a lumiphor for up-converting and/or down-converting radiation emitted by the semiconductor light emitting device.
A wide variety of lighting devices are known in the art including, for example, incandescent light bulbs, fluorescent lights and semiconductor light emitting devices such as light emitting diodes (“LEDs”). LEDs have the potential to exhibit very high efficiencies relative to conventional incandescent or fluorescent lights. However, significant challenges remain in providing LED-based lighting devices that simultaneously achieve high efficiencies, high luminous flux, good color reproduction and acceptable color stability.
LEDs generally include a series of semiconductor layers that may be epitaxially grown on a substrate such as, for example, a sapphire, silicon, silicon carbide, gallium nitride or gallium arsenide substrate. One or more semiconductor p-n junctions are formed in these epitaxial layers. When a sufficient voltage is applied across the p-n junction, electrons in the n-type semiconductor layers and holes in the p-type semiconductor layers flow toward the p-n junction. As the electrons and holes flow toward each other, some of the electrons will recombine with corresponding holes and, each time this occurs, a photon of light is emitted, which is how LEDs generate light. The wavelength distribution of the light generated by an LED generally depends on the semiconductor materials used and the structure of the thin epitaxial layers that make up the “active region” of the device (i.e., the area where the light is generated).
Most LEDs are nearly monochromatic light sources that appear to emit light having a single color. Thus, the spectral power distribution of the light emitted by most LEDs is centered about a “peak” wavelength, which is the wavelength where the spectral power distribution or “emission spectrum” of the LED reaches its maximum as detected by a photo-detector. The “width” of the spectral power distribution of most LEDs is between about 10 nm and 30 nm, where the width is measured at half the maximum illumination on each side of the emission spectrum (this width is referred to as the full-width-half-maximum or “FWHM” width). LEDs are often identified by their “peak” wavelength or, alternatively, by their “dominant” wavelength. The dominant wavelength of an LED is the wavelength of monochromatic light that has the same apparent color as the light emitted by the LED as perceived by the human eye. Because the human eye does not perceive all wavelengths equally (it perceives yellow and green better than red and blue), and because the light emitted by LEDs extends across a range of wavelengths, the color perceived (i.e., the dominant wavelength) may differ from the peak wavelength.
In order to use LEDs to generate white light, LED lighting devices have been provided that include several LEDs that each emit light of a different color. The different colors combine to produce a desired intensity and/or color of white light. For example, by simultaneously energizing red, green and blue LEDs, the resulting combined light may appear white, or nearly white, depending on, for example, the relative intensities, peak wavelengths and spectral power distributions of the source red, green and blue LEDs.
White light may also be produced by partially or fully surrounding a blue, purple or ultraviolet LED with one or more luminescent materials such as phosphors that convert some of the light emitted by the LED to light of one or more other colors. The combination of the light emitted by the LED that is not converted by the luminescent material(s) and the light of other colors that are emitted by the luminescent material(s) may produce a white or near-white light.
As one example, a white-light emitting lighting device may be formed by coating a gallium nitride based, blue light emitting LED with a yellow light emitting luminescent material such as a cerium-doped yttrium aluminum garnet phosphor (which has the chemical formula Y3Al5O12:Ce, and is commonly referred to as YAG:Ce). The blue LED produces an emission with a peak wavelength of, for example, about 460 nm. Some of blue light emitted by the LED passes between and/or through the YAG:Ce phosphor particles without being affected by the phosphor particles, while other of the blue light emitted by the LED is absorbed by the YAG:Ce phosphor, which becomes excited and emits yellow fluorescence with a peak wavelength of about 550 nm (i.e., the blue light is “down-converted” to yellow light). A viewer will perceive the combination of blue light and yellow light that is emitted by the coated LED as white light. This light is typically perceived as being cool white in color, as it includes a greater percentage of blue light which is in the lower half (shorter wavelength side) of the visible emission spectrum. To make the emitted white light appear more “warm” and/or exhibit better color rendering properties, red-light emitting luminescent materials such as CaAlSiN3 based phosphor particles may be added to the coating. Alternatively, the cool white emissions from the combination of the blue LED and the YAG:Ce phosphor may be supplemented with a red LED (e.g., an AlInGaP-based LED having a dominant wavelength of approximately 619 nm) to provide warmer white light, as is described in U.S. Patent Application Publication No. 2007/0170447.
Phosphors are a luminescent material that is widely used to convert single-color (typically blue or violet) LEDs into white LEDs. Herein, the term “phosphor” may refer to any material that absorbs light at one wavelength and re-emits light at a different wavelength in the visible or ultra violet spectrum, regardless of the delay between absorption and re-emission and regardless of the wavelengths involved. Thus, the term “phosphor” encompasses materials that are sometimes called fluorescent and/or phosphorescent. In general, phosphors may absorb light having emissions in a first wavelength distribution and re-emit light having emissions in a second wavelength distribution that is different from the first wavelength distribution. For example, “down-conversion” phosphors may absorb light having shorter wavelengths and re-emit light having longer wavelengths. In addition to phosphors, other luminescent materials include scintillators, day glow tapes, nanophosphors, quantum dots, and inks that glow in the visible spectrum upon illumination with (e.g., ultraviolet) light.
A medium that includes one or more luminescent materials that is positioned to receive light that is emitted by an LED or other semiconductor light emitting device is referred to herein as a “lumiphor.” With respect to LEDs, exemplary lumiphors include layers or “globs” that contain luminescent materials that are (1) coated or sprayed directly onto the LED, (2) coated or sprayed onto surfaces of a lens or other elements of the packaging of the LED, and/or (3) included within clear encapsulants (e.g., epoxy-based or silicone-based curable resin or glass or ceramic) that are positioned on or over the LED. A lumiphor may include one or multiple types of luminescent materials. Other materials may also be included within a lumiphor such as, for example, fillers, diffusants, colorants, or other materials that may improve the performance or cost of the material. If multiple types of luminescent materials are provided in a lumiphor, they may, for example, be mixed together in a single layer or deposited sequentially in successive layers.